Degassing history of a mid-ocean ridge rhyolite dome on the Alarcon Rise, Gulf of California

Abstract:

A 2350 meter deep rhyolite lava dome and surrounding intermediate-mafic complex on the Alarcon Rise mid-ocean ridge in the Gulf of California was sampled extensively during a 2012 MBARI expedition. The dome is predominantly composed of sparsely vesicular (<10%) obsidian with local deposits of pumiceous breccia. Pumiceous lapilli comprise highly vesicular (40-60%) fracture networks that separate non-vesicular obsidian “pseudoclasts”. Textures and major element geochemistry suggest that both lithologies originated from the same magma that formed the majority of the dome. This is corroborated by comparable major element compositions (~75% SiO₂) and near-equilibrium phenocryst assemblages including olivine (Fo¹⁰) and plagioclase (An¹⁷). Attenuated total reflectance (ATR) and transmission FTIR spectroscopy was used to measure H₂O concentrations in olivine and plagioclase melt inclusions as well as host glasses (CO₂ was below detection, <30 ppm). Rhyolite host glass contains 1.5-2.0 wt% H₂O, similar to nearby andesite and dacite. These concentrations agree with saturation limits for H₂O (1.7%) at the depth of Alarcon Rise, but are slightly less than what is predicted by fractional crystallization modeling. Melt inclusions from plagioclase and olivine in rhyolite contain a maximum of 3.5-4.5% H₂O suggesting that up to 3.0% H₂O exsolved into bubbles during a 3 km ascent. Hydrostatic pressures (23 MPa) at the eruptive vent would have permitted 53% vesiculation in agreement with petrographic observations. Although ~50% vesiculation and exsolved H₂O contents of 3.0 wt% are less than the ideal threshold for magmatic fragmentation, the presence of highly vesicular ash particles representing fragmented pumiceous breccia argues otherwise. We posit that decoupled volatiles from a deeper magma body migrated through fracture networks to the surface causing mild explosivity.